It is "reverse integration". Install the os onto a partition (don't enter product key ). As soon as you see oobe, hit ctrl+shift+f3 and it reboots into audit mode ( there is a sysprep window in the middle of the screen, leave that as it is for the moment ). Run Ricktendo's 4.5 slim installer , reboot ( it will go back into audit mode ) and leave it a few mins to do the ngen stuff. Then , in the sysprep window. select "Enter OOBE", tick the "generalise" box, select "shutdown", then click OK. Boot into another os or winpe and capture that partition into install.wim with maximum compression.
Someone wrote an addon-patch for "Win Toolkit" which will integrate Net Framework 4.5 (with all updates) in a Win7 WIM. It's not something official from Microsoft, but works just fine. (Sorry, can't post links yet, so google for "Wincert WinToolkit")